This invention relates to a powder composition which serves as a sintering material for a modified barium titanate semiconductive ceramic having a positive temperature coefficient of resistance and a method of producing the ceramic by using the powder composition.
It is well known that barium titanate ceramics can be rendered semiconductive by using a suitable dopant element such as Nb, Sb or a rare earth element such as Y, La or Ce. Barium titanate semiconductive ceramics are relatively low in specific resistance at normal temperature and, above the Curie temperature, have a high positive temperature coefficient (PTC) of resistance.
Usually the Curie temperature of a barium titanate semiconductive ceramic lies at about 120.degree. C., and it is known that the Curie temperature can be shifted to higher temperatures by partial substitution of Ba by Pb and to lower temperatures by partial substitution of Ba by Sr and/or Ca or by partial substitution of Ti by Zr and/or Sn. Sometimes the substitutions of Sr, Ca, Zr and/or Sn are made for the purpose of improving the electrical characteristics of the semiconductive ceramic.
Furthermore, there are proposals of additives to barium titanate base semiconductive ceramics for the purpose of stabilizing the important characteristics or desirably varying the temperature-resistance characteristics above the Curie temperature. For example, U.S. Pat. No. 3,441,517 shows the addition of Cu or Fe, and U.S. Pat. No. 4,101,454, JP 53-29386, JP 54-10110 and JP 63-28324 show the addition of Mn and Si or SiO.sub.2.
Barium titanate base semiconductive ceramics or PTC thermistors are used as self-regulating constant temperature heater elements, current limiting elements, temperature controlling elements, etc. For these uses the semiconductive ceramics are often required to be very low in specific resistance at normal temperature. However, from a practical point of view, barium titanate base semiconductive ceramics desirably low in specific resistance have not yet been developed because lowering of the specific resistance by known measures is accompanied by great degradation of the temperature-resistance characteristics and/or great lowering of the breakdown voltage. For example, when the specific resistance of conventional barium titanate base PTC ceramics is lowered to about 5.OMEGA..cm the temperature coefficient of resistance lowers to about 7 (%/.degree.C.), and the breakdown voltage lowers to the extent of about 30 V/mm (M. Nishii, Electronic Ceramics (a Japanese Journal), May, 1988, pp. 22-27).